Five model polydimethylsiloxane (PDMS) networks were obtained by hydrosilation of a difunctional vinyl-terminated PDMS prepolymer with a SiH containing crosslinker. Viscoelastic experiments were performed in order to study the influence of molecular parameters on the dynamic properties at the sol-gel threshold. Critical parameters were determined close to and above the sol-gel threshold. The results obtained suggest that the critical exponents depend on the chemical structure of the incipient networks.